This system minimizes the primary air and duct system from the main air handler since the fan-powered box does the local zone recirculation in the room/space. The main air handler is basically reduced to a dedicated outdoor air system (DOAS) supplying the required ventilation air with no recirculation or return air (climate dependent for de-humidifcation requirements and cooling load vs ventilation air requirements - many installations use recirc air for reheat after dehumidification)
The primary DOAS air is then sized for both the cooling load and the minimum ventilation air so the VAV part of the box modulates the cool primary supply air to meet the load while the fan-powered recirc air provides reheat capability. During unoccupied mode, the main AHU is off and the fan-powered box cycles on/off on a call for heating primarily at nights and weekends.
Most of my experience is with re-heat only fan-powered systems, being in a northern climate. Others on this board who design these for southern hot'n'humid climates may have more to say on this.
Typically fan assisted VAV boxes are used to increase air circulation and movement and seem to be becoming less popular as they use energy for generally little gain. With correct selection of diffusers it is possible to obtain sufficient induction and air movement using normal VAV boxes.
It is used in place of perimeter finned tube radiation to heat the perimeter spaces even at night when the main AHU is off. It draws ceiling plenum air which is relatively hotter than the occupied space below, since hot air rises. The system required plenum return air ceiling or no ceiling installation. With plenum return ceiling, wiring within must be plenum rated.
Discharge air temperature must not exceed about 90°F so discharge heated air can mix with cool room air without stratification.
The system cost less than wall to wall enclosure finned tube radiation system.
Of course that is if heating is required, in the climate I design for heating is only just required in the depths of winter when you might get to single digit (Celsius) temperatures at night!
If you have already spent the money on a VAV zoning system and you have a need for heat on the exterior zones the cost to install parallel fan powered boxes is an incremental cost. You've already paid for the VAV air handler, the controls, the VAV box, and the reheat.
You now have the added benefit of not needing as much reheat since, as lilliput1 said, you are reheating the relatively warmer plenum air. Which means smaller heating coil requirements. (75 F to 90 F vs 55 F to 90 F) There is a payback both in first cost (smaller coils, smaller gauge wire or smaller hot water pipe and boiler), and energy usage when heating during occupied times.
When you look at unoccupied times you don't have to run a 20, 30, or possibly 50 HP fan motor to heat one exterior zone that needs heat and can just run the VAV box's 1/4 HP motor and recirculate the air in that one zone.
Those are the reasons I would "waste" the money on the boxes .
I always figured heat from lights that warm up a plenum ceiling space could get sent back down to the space with them, probably an advantage in heating up North.
Take the "V" out of HVAC and you are left with a HAC(k) job.
AbbyNormal in a parallel configuration that heat would only get sent back down when it was needed as the fan only runs when you need heat. A series box is a whole different animal though.
